According to a method described, e.g., in published German patent document DE 44 20 962, self-supporting MEMS structures in silicon (MEMS=“microelectromechanical structures”) are produced by a combination of anisotropic and isotropic etching techniques, and so-called trenches or deep structures having vertical side walls are initially anisotropically etched into a silicon substrate using a reactive plasma. Once the desired etching depth is achieved, then, following a longer passivating step for depositing a Teflon-like film, a longer etching step is performed, during which first a Teflon polymer that formed on the etching ground of the trench is removed by ionic bombardment and subsequently the self-supporting MEMS structures to be produced are isotropically undercut. During this isotropic undercutting, the vertical side walls of the trenches are protected from an etch attack by the previously applied Teflon-like film. According to published German patent document DE 44 20 962, the isotropic undercutting of the MEMS structures produced proceeds in the silicon substrate material on a purely time-controlled basis.
For depositing the Teflon-like film, a fluorohydrocarbon having a fluorine to carbon ratio that is as low as possible, e.g., 2:1, may be used, fluorohydrocarbons such as C4F6, C5F8, C4F8 or C3F6 being suitable for this purpose. A process gas that generally etches isotropically and that provides fluorine radicals, such as SF6, may be used for etching silicon.
Published German patent document DE 198 47 455 describes limiting isotropic undercutting vertically with the aid of buried oxide layers. Particularly, an etch attack on the MEMS structures is prevented by an intermediate oxide separating the MEMS structure from a sacrificial layer made of silicon. Moreover, it is also described that, instead of using fluorine radicals from a plasma discharge, isotropic undercutting can also be performed using spontaneously and plasmalessly silicon-etching fluorine compounds such as XeF2, CIF3 or BrF3. Following adsorption and chemisorption on a silicon surface, these compounds spontaneously split off fluorine radicals, which results in an isotropic etching removal of the silicon through the formation of silicon tetrafluoride. At the same time there is a very high selectivity vis-à-vis non-silicon materials such as Teflon-like passivating layers or passivating layers of other compositions. In particular, photo-resist is attacked by CIF3 in a way that is practically not measurable, so that on the one hand it is particularly easy to passivate parts of a silicon wafer that are not to be etched, while on the other hand very thin passivating layers are already sufficient to ensure complete protection against an etch attack. In many cases, the so-called “native” silicon oxide normally present anyway on silicon surfaces is already able to withstand a chlorine trifluoride attack for several minutes without the silicon underneath it being etched.
A further aspect in the use of CIF3 (and in a limited way also BrF3) is the generally low reactivity of these highly oxidizing fluorine compounds vis-à-vis silicon, which has the result that an etching performed with them proceeds in a reaction-controlled manner over a wider parameter range and is not limited by a mass transport. This being the case, large undercutting widths can be achieved without a reduction of the etch rate due to a growing aspect ratio of the undercutting channels. With the aid of these gases it is possible, for example, for the lateral extension of the undercutting channels to amount to a multiple, e.g., 100 to 1000, of their vertical extension.
Finally, isotropic etching using these gases occurs entirely without ion action, which is of great advantage with regard to the desired isotropy and selectivity vis-à-vis non-silicon materials.
On the whole, CIF3 is in many respects an ideal gas for the selective removal of silicon or porous silicon within the scope of a sacrificial layer technique in that it allows in a very simple manner for the creation of self-supporting membrane structures with minimal restrictions on the freedom of design. In many cases, even a single etch opening is sufficient in order to achieve a complete undercutting of a membrane area on porous silicon for example.
On the other hand, the disadvantage in the use of CIF3, and with qualifications also of BrF3, is the fact that it very easily spreads unchecked via microchannel structures or even nanochannel structures, so that there is a great danger of it creeping extremely quickly behind deposited passivating layers, for example Teflon-like side wall passivating layers described in published German patent document DE 42 41 045, via micro- or nanocracks in the interface area to the silicon.
Thus, a Teflon-like plasma passivating layer produced from C4F8 or C3F6 has sufficiently many microchannels or nanochannels in the interface area to the silicon below so as to expose, starting from a single opening, for example on the etching ground of the initially produced trench, the entire MEMS structure over a large area to a CIF3 etch attack in spite of the deposited Teflon-like passivation. A typical defect pattern in this regard is that structures, in which the etching ground was previously cleared of a passivating teflon polymer with the help of ions as described in published German patent document DE 42 41 045, are massively attacked by etching on the entire, generally passivated side wall surface, while structures, in which the etching ground had a remaining, few nanometers thick coating of a Teflon-like polymer, nowhere show signs of an etch attack, even after a long period. The cause of this problem are the microchannels or nanochannels, explained above, in the interface area between the Teflon passivation, which generally adheres poorly to silicon, and the silicon surface, which allow etching gas CIF3 access to the silicon at undesired places.
An object of the present invention is to provide a passivating layer on a silicon layer, as well as a method for producing such a passivating layer on a silicon layer, which achieves a reinforced passivation of the interface between the passivating layer and the silicon layer by preventing the formation of, or alternatively by closing, undesired microscale or nanoscale channels, through which otherwise an uncontrolled etch attack, particularly by an isotropically etching gas such as CIF3 or BrF3, would occur. In addition, it should be possible to integrate the method and the obtained layer system with the passivating layer into plasma etching methods using photo-resist masking for producing trenches and self-supporting structures.